(Image Credit: Sanford Police Department/AP Photo)

ABC News' Seni Tienabeso (@SeniABC) reports:

Lawyers for George Zimmerman believe that a newly released color photo of injuries their client sustained during his fatal altercation with Trayvon Martin and an alleged conversation involving Martin's father create doubts about the strength of the murder case against the former neighborhood watch captain.

The color photo, which was released months ago in black-and-white form, shows Zimmerman's swollen and bruised nose with blood dripping down his mouth. The image was released on the Zimmerman defense website, gzlegalcase.com. His attorneys released the image publicly Monday, along with a separate motion asking Sanford police investigator William Irwin to take another deposition because of comments he made that came to their attention after his first deposition.

Zimmerman's attorneys say in the document that they learned after initially taking Irwin's deposition that he told Sanford police Captain Robert O'Connor about overhearing a potentially damaging conversation between Martin's father and lead investigator Chris Serino.

In the alleged conversation, Irwin, who took the voice stress test on Zimmerman after his deadly confrontation with Martin, said he heard Tracy Martin, the unarmed teen's father, tell Serino that the still unidentified screams in those now-infamous calls did not belong to his son.

In the stress-test and throughout various interrogations, Zimmerman maintained that he acted in self-defense when he shot and killed Martin because he believed the teen, who he says attacked him, was going to cause severe bodily injury to him if he did not do something. Zimmerman maintains that he was screaming for help, and shot Martin, 17, only after being repeatedly pummeled by the teen, who was walking back to his father's girlfriends home from 7-Eleven after buying Skittles and tea.

Zimmerman's attorneys call the conversation in the motion "significant," saying "the primary issue in this case, if not the sole issue to be decided either by this court at a Self Defense Immunity hearing or by a jury at trial is whether Zimmerman was acting in self-defense."

The elder Martin has disputed this police account in the past with both he and the teenager's mother saying that they believed the cries for help belonged to their son, and any misstatements happened during the early confusion of not knowing what happened to their son.

Audio test of the screams on the 911 call, which was placed by a neighbor near the scene, have not conclusively linked the cry for help to either Zimmerman or Martin, casting another layer of doubt in this story.

This latest information was released as Zimmerman's defense continues to gather evidence it will use in a "Stand-Your-Ground" self-defense hearing next year that, if the court sides with Zimmerman, could mean he never actually goes to trial for second-degree murder, as charged.

In the past few weeks, his defense has been interviewing officers involved in the early investigation, many of whom believed there was insufficient evidence to arrest Zimmerman.

Serino, the lead investigator who recommended charges be filed early on, hired Casey Anthony attorney Jose Baez in a highly unusual move last week. Audio and video taken days after the shooting showed that Serino was tough on Zimmerman during several interrogations, openly questioning his self-defense claim.

"You ever hear of Murphy's law," Serino asked during a Feb. 29 audio interrogation. "This person was not doing anything bad."

Serino then questioned the extent of Zimmerman's injuries, telling him that they didn't appear consistent with someone involved in a life-or-death struggle

A few weeks later, in a March 13 affidavit, Serino wrote, "I believe there exists probable cause … charging George Zimmerman with Manslaughter."

In a separate interview with the FBI, however, Serino told agents that he was feeling the pressure to file charges against Zimmerman.

Zimmerman's defense has already listed several members of the Sanford police department as potential witnesses, and has stated it will claim that public pressure and a lack of evidence led to charges being filed against their client.

All this activity comes as Zimmerman remains in hiding and low on cash. A message on his defense website saying that he would sign "Thank You Cards" to supporters who donated to his fund has been publicly ridiculed.

Donations to his fund have dwindled in recent months, and the once-large reserve he had never fully recovered from the new bail he posted after being re-arrested for making false claims in court about the state of his finances.

Those claims also briefly landed his wife, Shellie, who still faces perjury charges, behind bars.